Neither the SM420 nor the SM465 has an overdrive gear. For this reason we optioned the Advance Adapters Saturn overdrive to help drop the rpm for highway cruising. Because the transfer case has both a front and rear passenger offset output, the power from the engine must make a 90-degree turn on its way out to the driveshafts. The Saturn overdrive mounts on the back of the transfer case so the power runs through it in either two- or four-wheel drive, and high or low range.